1. Field of the Invention
The present invention relates to a signal generator with a DDS synthesizer instead of a down mixer.
2. Discussion of the Background
Modern signal generators are required for a wide range of applications. In order to be suitable for the maximum possible number of such measuring applications, the signal generator must provide an extremely broad HF frequency range with very high frequency resolution. Currently-available signal generators are therefore specified with a frequency range from a few kHz up to 6 GHz. Special microwave generators even supply HF signals from typically 10 MHz into the high-frequency range up to 60 GHz. However, these are not generally used for the above-named applications because of the relatively higher price.
A distinction can be made between two classes of signal generators. On the one hand, there are digitally-modulated signal generators, which generate complex IQ-modulated HF signals, as used nowadays primarily in mobile telephone systems. On the other hand, there are analog-modulated signal generators, which offer the classical analog-modulation methods such as frequency/phase modulation, amplitude modulation and pulse modulation.
Frequency generation in currently-available signal generators is implemented by means of synthesizers. Two methods are used to generate the required high level of frequency resolution of the synthesizer (within the range of a few μHz or even lower). On the one hand, there is the fractional-N-method, in which a frequency divider in the return path of a phase locked loop (PLL) or in the reference path upstream of the phase detector is varied dynamically with regard to the division factor in such a manner that, averaged over time, fractional division ratios of any required resolution are obtained. The second method is a synthesizer based on the principle of direct digital synthesis (the direct digital synthesis (DDS) principle), wherein the DDS generates either the reference signal of a PLL or can be used in the return path of a PLL as a type of fractional divider.
In this case, the HF signal of the synthesizer is supplied by voltage-controlled oscillators (VCO), which are connected via a PLL as described above to the constant quartz reference of the generator. These generally cover one frequency octave, the so-called basic octave. With subsequent frequency division, multiplication and mixing, the HF output signal of the signal generator is generated within a frequency range of the required magnitude.
VCOs generally supply HF signals within a frequency range from, for example, 750 MHz to 1500 MHz. The frequency range 1500 MHz to 3000 MHz, in this example, is obtained through simple frequency doubling. The range from 3000 MHz to 6000 MHz is obtained by further doubling. The frequency range 375 MHz to 750 MHz is obtained by frequency division by 2, and the range 187.5 MHz to 375 MHz is obtained by division by 4, and so on.
Digital frequency dividers have the disadvantage that they provide square-wave signals at their output, which must subsequently be subjected to complex low-pass filtering, in order to obtain a sinusoidal HF output signal with a low harmonic. When using non-adjustable low-pass filters, at least two filters of this kind are required for each octave.
An extremely high expenditure on division and filtering would be required, if it were necessary to generate a sinusoidal HF output signal by means of frequency division up to the kHz range, which would incur large costs determined by the need for a frequency divider and filter at every stage.
Hitherto, it has therefore been conventional to generate the lower frequency range with a down mixer. This requires a relatively higher-frequency oscillator signal, which is derived from the quartz reference of the signal generator. The derivation is implemented via a phase locked loop. With this known method, signal frequencies are generated on the basis of an oscillator frequency using a variable frequency divider and a phase difference formation within a closed loop arrangement.
Typical configurations for generating the lower frequency range are based upon a frequency of the basic octave. In this context, a fractional divider is generally used:
LO signal: 1000 MHz
Down mixer range: 9 kHz to 100 MHz
HF signal of the down mixer: 1000.009 to 1100 MHz (contained in the basic octave of the synthesizer).